What is Reflectance Spectroscopy?

In brief, spectroscopy is the study of the electromagnetic spectrum
as emitted or
reflected by various materials. Reflectance spectroscopy focuses on the
radiation reflected by different materials.

Anyone with a little familiarity with rocks can at a glance
recognize a piece of basalt or limestone. In this case, color
and texture give the observer immediate
clues to the rock type. At this simple level a basalt is easily recognized
due to its black color (low albedo) and smooth texture (fine-grained).
Limestone is buff-white in color (high albedo) and also smooth or
fine grained. Sandstones are sometimes red and have noticeable grains.
Albedo is simply a measure of the amount of light that is reflected
from a surface (rock, mineral, soil, etc). A dark rock only
reflects a small portion of sunlight that hits it, thus it
has a low albedo. You can think of albedo as the percentage
of light that is reflected from a surface versus the amount
that shines on the that surface.

However a slate may look very similar to a basalt --
black and fine grained. Sometimes limestone can also look like
basalt - when it has small amounts of carbon it too is fine grained
and black. Spectroscopy allows scientists to recognize
different types of rocks that may initially look or feel the same,
or which are too far away to be touched at all, based on subtle
differences in how material reflects light across a broad wavelength
range.

Basalt

Limestone

Black Limestone

Sand Stone

Slate

Electromagnetic radiation consists of energy -
the shorter the wavelength, the more energetic
the light is. Electrons in the rock can absorb this energy,
but only at specific
energy levels characteristic of a specific atom or molecule. Thus,
minerals are more likely to absorb radiation at certain energies, and so
only at certain wavelengths. Specific molecules and minerals absorb
characteristic wavelengths of radiation, allowing scientists to identify
them through their spectra, much as people can be identified through
their fingerprints. Energy that is not absorbed is reflected, this
reflected energy is what is measured with the spectrometer. We can easily
determine the amount of absorption by inspecting dips in the
reflectance spectrum.

Reflectance Spectrum of Martian Meteorite ALH84001

The energy reflected by a rock (or from a planetary surface)
can be broken into very fine divisions over a wavelength range much
wider than that seen by the human eye. This spectrum can be examined
using a spectrometer, which can gather data about UV light and IR
radiation in addition to the visible ranges (energy, or light,
that we can see.) In
that spectrum, we should see drops in reflection at certain points
where the light is being absorbed by electrons. The points at which
these dips, or "absorption bands," occur are characteristic of certain
ions, molecules, and minerals. By comparing the position and
depth of the absorption bands of
an unknown material to the absorption bands of known minerals, we can
determine the mineral content of that material. Reflectance spectroscopy
allows scientists to estimate the chemical content of planets and
asteroids without analyzing actual rock samples.

For the case of basalt as seen by the human eye strong absorptions occur
more or less evenly over the whole visible spectrum, thus its black color.
In the case of limestone absorptions are weak across the entire visible
spectrum, thus its generally light appearance. In the case of basalt atoms
of iron occur in several minerals (pyroxenes and olivines) that have a
distinctive absorption the near-infrared (~0.950 µm), just beyond the range
of the human eye (~0.4-0.7 µm). The exact wavelength center and depth
of this absorption indicates the type of pyroxene and the amount of
pyroxene relative to olivine (especially with help of another absorption
at about 2.0 µm). These two absorptions are often referred to as
1-micron and 2-micron (see arrows 1 and 2 in spectra figure above)
features. Unfortunately spectroscopists often mix wavelength units
leading to confusion amongst those not familar with this tradition. In the
above plot the wavelenghts are shown in nanometers, but here we are
referring to microns (1000 nm equals 1 micron)!

The spectral range useful for reflectance spectroscopy for planetary
geologists is limited by the wavelengths of sunlight that the planets
reflect and by the wavelength of thermal radiation emitted.
Planetary bodies eventually emit
the energy they have absorbed from light in the form of longer
wavelength IR radiation. Reflectance spectroscopy is limited to
spectral ranges where the level of reflected light is much greater
than the level of emitted light. In total, the useful region for
reflectance spectroscopy is usually about .3 to 4.0 µm.